Hydrolytic Degradation of Dental Compositesand

Effects of Silane-Treatment and Filler Fraction on Compressive Strength and Thermal Expansion of Composites

From the Department of Dental Technology Faculty of Odontology

University of Umeå S-901 87 Umeå, Sweden

Department of Dental Biomaterials College of Dentistry

University of Florida Gainesville, Florida 32610, USA

andScandinavian Institute of Dental Materials

Forskningsveien 1 Oslo 3, Norway

A K A D E M I S K A V H A N D L I N G

som med vederbörligt tillstånd av odontologiska fakulteten vid Umeå universitet

för vinnande av odontologie doktorsexamen offentligen försvaras i Sal B, 9 tr

Odontologiska kliniken måndagen den 28 maj 1984 kl. 09.00

av

KARL-JOHAN M. SÖDERHOLMLeg. tandläkare, f il .k a n d ., m . ph il.

ABSTRACT

Söderholm, Karl-Johan, M. 1984. H ydrolytic degradation of dental composites and effects of silane-treatment and filler fraction on compressive strength and thermal expansion of composites. Umeå University Odontological Dissertations No 19, ISSN 0345-7532.

Some researchers have suggested that the weakest link of dental composites is the filler-matrix bond. However, due to incompleteness of information dealing with this bond and its stability in a humid environment, it was considered desirable to investigate the effect of water on this region, as well as the influence of filler bonding and filler fraction on compressive strength and thermal expansion.

Experimental composites containing different filler fractions of either silane-treated or untreated fillers were made. Compressive strength and coefficient of thermal expansion were determined using routine methods, while the hydrolytic degradation was investigated by measuring changes in concentrations of elements in the storage water using atomic absorption spectrophotometry. Scanning electron microscopic investigations were made on fractured samples.The diffusion coefficient of a representative resin system was determined gravi- metrically. Seven commercial composites were investigated regarding hydrolytic degradation. The filler compositions of these composite materials were determined by emission spectroscopy or energy-dispersive x-ray analysis before storage in distilled water. This water was replaced and analyzed m onthly using plasma spectrophotometry or atomic absorption spectrophotometry.After completed water storage the samples were fractured and investigated by use of scanning electron microscopy.

From the results of these studies the following conclusions were drawn:1. The compressive strength of composites changes linearly with increased filler

fraction. Contrary to bonded fillers, composites containing unbonded fillers lost strength with increased filler fraction.

2. W ater diffuses through the polymer matrix and attacks the filler particles. This degradation is most pronounced for untreated fillers containing glass modifying elements such as sodium, barium and strontium.

3. The resin, used as a matrix, influences the speed with which the hydrolytic degradation of the filler proceeds.

4. The hydrolytic degradation of the filler seemed to be associated with micro-crack formation occurring in the matrix. Of the investigated composites, the micro-filled resin showed the lowest frequency of such crack formations.

5. The coefficient of thermal expansion decreases linearly with increased filler fraction. Silane treatm ent did not influence this coefficient.

6. Using a simplified model to predict stresses in a particle filled composite indicates that rather high stress levels are induced in the polymer matrix due to polymerization shrinkage. This shrinkage induces radial compressive and tangential tensile stresses with respect to the filler surface. Increased filler fraction increases the tangential tensile stresses but reduces the compressive radial stresses.

Key words: Internal frictional theory, filler-matrix debonding, stress-corrosion, stress- influenced diffusion, micro-crack formation, crazing, opacity, wear, osmotic pressure, hoop stresses.

K-J. Söderholm, Departm ent of Dental Biomaterials, College of Dentistry, University of Florida, Gainesville, FL 32610, U. S. A.

ABSTRACT

Söderholm, Karl-Johan, M. 1984. H ydrolytic degradation of dental composites and effects of silane-treatment and filler fraction on compressive strength and thermal expansion of composites. Umeå University Odontological Dissertations No 19, ISSN 0345-7532.

Some researchers have suggested that the weakest link of dental composites is the filler-matrix bond. However, due to incompleteness of information dealing with this bond and its stability in a humid environment, it was considered desirable to investigate the effect of water on this region, as well as the influence of filler bonding and filler fraction on compressive strength and thermal expansion.

Experimental composites containing different filler fractions of either silane-treated or untreated fillers were made. Compressive strength and coefficient of thermal expansion were determined using routine methods, while the hydrolytic degradation was investigated by measuring changes in concentrations of elements in the storage water using atomic absorption spectrophotometry. Scanning electron microscopic investigations were made on fractured samples.The diffusion coefficient of a representative resin system was determined gravi- metrically. Seven commercial composites were investigated regarding hydrolytic degradation. The filler compositions of these composite materials were determined by emission spectroscopy or energy-dispersive x-ray analysis before storage in distilled water. This water was replaced and analyzed m onthly using plasma spectrophotometry or atomic absorption spectrophotometry.After completed water storage the samples were fractured and investigated by use of scanning electron microscopy.

From the results of these studies the following conclusions were drawn:1. The compressive strength of composites changes linearly with increased filler

fraction. Contrary to bonded fillers, composites containing unbonded fillers lost strength with increased filler fraction.

2. Water diffuses through the polymer matrix and attacks the filler particles. This degradation is most pronounced for untreated fillers containing glass modifying elements such as sodium, barium and strontium.

3. The resin, used as a matrix, influences the speed with which the hydrolytic degradation of the filler proceeds.

4. The hydrolytic degradation of the filler seemed to be associated with micro-crack formation occurring in the matrix. Of the investigated composites, the micro-filled resin showed the lowest frequency of such crack formations.

5. The coefficient of thermal expansion decreases linearly with increased filler fraction. Silane treatm ent did not influence this coefficient.

6. Using a simplified model to predict stresses in a particle filled composite indicates that rather high stress levels are induced in the polymer matrix due to polymerization shrinkage. This shrinkage induces radial compressive and tangential tensile stresses with respect to the filler surface. Increased filler fraction increases the tangential tensile stresses but reduces the compressive radial stresses.

Key words: Internal frictional theory, filler-matrix debonding, stress-corrosion, stress- influenced diffusion, micro-crack formation, crazing, opacity, wear, osmotic pressure, hoop stresses.

K-J. Söderholm, Department of Dental Biomaterials, College of Dentistry, University of Florida, Gainesvilie, FL 32610, U. S. A.

4

CONTENTS

1. PREFACE 6

2. INTRODUCTION 7

General background

The f i r s t commercial composites

The tra n s it io n from amalgam to composite resins

Development trends

Present shortcomings regarding composites

Summing-up

3. OBJECTIVES 19

4. EXPERIMENTAL PROCEDURES AND RESULTS 20

I . RELATIONSHIP BETWEEN COMPRESSIVE YIELD STRENGTH AND 20

FILLER FRACTIONS OF PMMA COMPOSITES

M ateria ls and methods

Results

I I . DEGRADATION OF GLASS FILLER IN EXPERIMENTAL COMPOSITES 22

M ateria ls and methods

Results

I I I . WATER SORPTION IN A BIS-GMA/TEGDMA RESIN 25

M ateria ls and methods

Results

IV. LEACHING OF FILLERS IN DENTAL COMPOSITES 27

M ateria ls and'methods

Results

V. HYDROLYTIC DEGRADATION OF DENTAL COMPOSITES 29

M ateria ls and methods

Results

5

VI. INFLUENCE OF SILANE-TREATMENT AND FILLER FRACTION ON 32

THERMAL EXPANSION OF COMPOSITE RESINS

M ateria ls and methods

Results

5. DISCUSSION 34

6. SUMMARY & CONCLUSION 50

7. ACKNOWLEDGEMENTS 53

8. REFERENCES 55

6

PREFACE

This t h e s i s i s based on the fo l lowing pa per s , which wi l l be r e f e r r e d

t o by t h e i r Roman numerals:

I. Söderholm, K-J . : R e la t i o n s h ip between compressive y i e l d s t r e n g t h

and f i l l e r f r a c t i o n s o f PMMA compos i tes. Acta Odonto l . Scand.

1982, 40:145-150.

I I . Söderholm, K-J . : Degradat ion of g l a s s f i l l e r in exper imenta l

composi tes . J . Dent. Res. 1981, 60:1867-1875.

I I I . Söderholm, K-J . : Water s o r p t i o n in a BIS-GMA/TEGDMA r e s i n . J .

Biomed. Mater. Res. 1984, 18:271-279.

IV. Söderholm, K-J . : Leaking of f i l l e r s in denta l compos i tes. J .

Dent. Res. 1983, 62:126-130.

V. Söderholm, K - J . , Zigan, M., Ragan, M., F i s c h l s c h w e i g e r , W., and

Bergman, M.: Hydro ly t ic degr ada t io n of denta l compos i tes . J .

Dent. Res. ( submi t ted f o r p u b l i c a t i o n ) .

VI . Söderholm, K-J . : I n f luence of s i l a n e t r e a t m e n t and f i l l e r

f r a c t i o n on thermal expans ion of composite r e s i n . J . Dent. Res.

( s ub m i t t ed f o r p u b l i c a t i o n ) .

★Copyright Apri l 10, 1984. Repr in ted by permission of John Wiley &

Sons, Inc .

7

INTRODUCTION

General Background

The idea to make a to o th co lo red r e s t o r a t i v e m a te r ia l by mixing

i n o r g a n ic f i l l e r p a r t i c l e s with a c o ld - c u r in g r e s i n da tes back to 1951

(Knock & Glenn, 1951). The j u s t i f i c a t i o n f o r adding f i l l e r was t o

reduce po ly me r i za t io n s h r in k a g e , thermal expans ion , and w a te r so r p t i o n

of those r e s i n based too th co lo red m a t e r i a l s a v a i l a b l e a t t h a t t ime

( Pa f f enba rg e r e t a l . , 1953; Rose e t a l . , 1955). However, t h i s p r e c u r

sor to our modern dental composi tes never became a su c c e ss . One

p o s s i b l e ex p lan a t i o n f o r i t s poor success might have been t h a t t h i s

m a te r ia l was in t roduced a t a time when d e n t i s t s s t a r t e d to exp e r ie n c e

some c l i n i c a l f a i l u r e s with the u n f i l l e d r e s i n s (Caul & Schoonover,

1953; Kramer, 1954; Hedegård, 1955). T h e re fo re , r a t h e r than r i s k and

f a i l u r e s by u'sing the new r e s i n - b a s e d , to o t h co lo red r e s t o r a t i v ei

m a t e r i a l , most d e n t i s t s r e tu r n e d to the use of s i l i c a t e cements w i th

which they had a t l e a s t some prev ious exper ience ( P a f f e n b a r g e r , 1972).

Though most d e n t i s t s r e tu r n e d to the use of s i l i c a t e s , t h e r e were

some c l i n i c i a n s and r e s e a r c h e r s who cont inued to t r y to f i n d new ways

to use the co ld -c u re d r e s i n s c l i n i c a l l y . A new concept in t r o d u ced

d ur in g t h i s per iod was a method f o r bonding r e s i n s to the enamel by

e tch in g the enamel with an ac id (Buonocore, 1955). Epoxy r e s i n was

proposed a t about th e same t ime as an exper imenta l m a te r i a l s i nce i t

was b e l i eved t h a t t h i s type of r e s i n had the p o t e n t i a l to bond t o

t o o t h s t r u c t u r e (Bowen, 1956). An in o rg an ic f i l l e r was added to the

epoxy r e s i n to reduce shr inkage and to improve bonding along t h e

e n a m e l - r e s in i n t e r f a c e . In 1963, Bowen a l s o in t r odu ced th e concept of

8

a coupling agent to achieve a chemical bond between r e s i n and f i l l e r s .

Although the l a b o r a t o r y r e s u l t s wi th t h e s e exper imental m a t e r i a l s were

very promis ing , Bowen soon r e a l i z e d t h a t the epoxy r e s i n he was

i n v e s t i g a t i n g could not be used c l i n i c a l l y as i t d id not cure in a

m o is t environment (Bowen & Molineaux, 1969). To ach ieve cur ing

i n t r a o r a l l y , a new r e s i n was formula ted by l i n k in g two methyl

m e t h a c r y l a t e molecules to the two epoxy groups of the epoxy r e s i n

molecule . This m o d i f i c a t i o n of the epoxy r e s i n was done e i t h e r by t h e

r e a c t i o n of bisphenol A and g l yc idy l m e th a c ry l a t e or by th e r e a c t i o n

of the d ig ly c id y l e t h e r of bisphenol A and m e th a c ry l i c a c id . Each o f

t h e s e r e a c t i o n s r e s u l t e d in th e so c a l l e d BIS-GMA monomer (Bowen 1963,

1965). Through t h i s approach, the epoxy r e s i n was conver ted to a

d i m e t h a c r y l a t e which could be polymerized in the same way as the co l d -

cured methyl m e th a c r y l a t e . Some of the advantages of t h i s monomer

compared to p re v io u s ly -u sed methyl me th ac ry la te s in c lu de : l e s s

polyme r iza t ion s h r in k a g e , l e s s v o l a t i l i t y , and an inc rease d c r o s s l i n k

ing p o t e n t i a l dur ing p o l y m e r i z a t i o n . However, t he BIS-GMA monomer

proved to be ex tremely v i scous and i t was d i f f i c u l t to mix i t w i th

f i l l e r s wi thout i n c o r p o r a t i n g and r e t a i n i n g a s i g n i f i c a n t

c o n c e n t r a t i o n of vo id s . To reduce the v i s c o s i t y , the BIS-GMA monomer

was d i l u t e d wi th o th e r more f l u i d a c r y l a t e s such as t e t r a e t h y l e n e

glycol d i m e th a c ry l a t e before the s i i arie t r e a t e d f i l l e r s were added

(Bowen, 1965).

The F i r s t Commercial Composites

Although f a i l i n g to chemica l ly bond to to o th s t r u c t u r e , Bowen's r e s i n

has se rved as the mat r ix f o r most o f th e c u r r e n t composite m a t e r i a l s

9

(Asmussen, 1975; Ruyter & S j 0 v i k , 1981). The c l i n i c a l success o f t h i s

r e s i n for mu la t io n was r e a l i z e d soon a f t e r i t s i n t r o d u c t i o n in 1964 by

th e 3M Corporat ion as a commercial p r o d u c t , Addent 35 (Pe te r son e t

a l . , 1966). This success was a t t r i b u t e d to the c l i n i c a l d u r a b i l i t y o f

t h e composi tes compared to t h a t o f th e s i l i c a t e s (Bowen e t a l . , 1968;

Brauer, 1977). However, th e composi tes e x h i b i t e d some major c l i n i c a l

shor tcomings such as poor c o l o r s t a b i l i t y , a rough s u r f a c e which

enhanced plaque adhes ion , and u n s a t i s f a c t o r y wear r e s i s t a n c e

( M o s t e l l e r , 1972; E r i k sen , 1974; P h i l l i p s e t a l . , 1973). These

f i n d i n g s , combined wi th th e prev ious poor c l i n i c a l per formance o f

c o l d - c u r e d methyl m et hac ry la te m a t e r i a l s , would have been expec ted to

ad v e r se ly a f f e c t the marke ting and development of composite m a t e r i a l s .

F o r t u n a t e l y , t h i s s i t u a t i o n d id not occur because o f the i n t r o d u c t i o n

a t t h i s t ime of the ac id e tch techn ique f o r composite r e s i n s (B jo rv a tn

& Tornes , 1972). The su c c e s s fu l combinat ion o f the a c id e tch

technique and composite r e s i n s made i t p o s s i b l e to r e s t o r e t e e t h i n

ways p r e v io u s ly cons ide red imposs ib le and helped compos ites overcome

what could have become the f i r s t " c r i s i s " (Jordan e t a l . , 1977).

The T r a n s i t i o n from Amalgams to Composite Resins

Since the i n t r o d u c t i o n of the ac id e t ch t e c h n iq u e , re se a rc h a c t i v i t y

has i nc reased toward the development o f improved composi tes as well as

bonding agents (Bowen, 1979; Smith, 1982). This t r end i s r e l a t e d t o

th e numerous new c l i n i c a l t r e a t m e n t p o s s i b i l i t i e s t h a t a ro se from such

breakthroughs in dent a l m a t e r i a l s sc ie n c e . Improved wear r e s i s t a n c e

o f composi te r e s i n s could mean t h a t th e s e products would r e p la c e

denta l amalgam as th e most commonly-used r e s t o r a t i v e m a t e r i a l . S i nce

10

amalgams have cap tur ed approximate ly 75% of th e r e s t o r a t i v e ma ter ia l

market toda y , the economical as pe c ts o f such a t r a n s i t i o n a r e obvious.

A d isa dvan tage of dental amalgams i s t h a t a shor t age o f meta l s used in

amalgams ( t i n , s i l v e r , mercury and copper) could a r i s e in the n e a r

f u t u r e (Kusy, 1982). In c o n t r a s t , composi tes should have a b r i g h t e r

f u t u r e as the f i l l e r c o n s t i t u e n t used in these m a t e r i a l s i s a v a i l a b l e

in almost un l im i te d q u a n t i t i e s .

In a d d i t io n to the se impor tan t economical a s p e c t s , den tal compos

i t e s a l s o possess a c o l o r s i m i l a r to the t o o t h . As d e n t i s t r y i s

co n t in uous ly c o n c e n t r a t i n g on more-demanding e s t h e t i c re qu i r em en ts , we

can expec t an inc re as ed demand from the consumer f o r th e se

r e s t o r a t i o n s in the f u t u r e . Another t r e n d in fa vo r of composi tes i s

an i nc reased concern from the publ ic r eg ard in g mercury t o x i c i t y from

amalgam r e s t o r a t i o n s (S c h o l l e , 1983). The s c i e n t i f i c evidence i s

in c o n c l u s i v e to j u s t i f y such a t r a n s i t i o n to composi tes a t t he pr e se n t

t ime (Dauer & F i r s t , 1982). I n s t e a d , the e x t e n s iv e c l i n i c a l

e x p e r i e n c e with amalgam th e ra py gained dur ing th e l a s t ce n tu ry would

j u s t i f y the cont inued use of amalgams u n t i l r e se a rch has proven t h a t

composi tes a re a t l e a s t as good an a l t e r n a t i v e as amalgams (Skjf l r land,

1973; L e i n f e l d e r e t a l . , 1983; S tan ley e t a l . , 1979).

Development Trends

During th e l a s t t en y ea rs in c re a se d a t t e n t i o n has been focused on t h e

development of improved composi tes . Although r e se a rc h e f f o r t s have

markedly in c re a se d w i t h in th e p as t seve ra l y e a r s , the wear r e s i s t a n c e

o f composi tes has not improved s u b s t a n t i a l l y . These e f f o r t s have

11

r e s u l t e d in the a v a i l a b i l i t y of more products today than eve r b e f o r e ,

making i t q u i t e d i f f i c u l t to s e l e c t the most s u i t a b l e m a t e r i a l .

Several p u b l i c a t i o n s have r e c e n t l y summarized informa t ion r e g a rd in g

t h e orga nic and ino rga nic components in most commercial composite

products (Asmussen, 1975; Ruyter & Sj 0 v ik , 1981; Zinck e t a l . , 1982).

These papers show t h a t the most commonly-used r e s i n component i s 2,

2 -b is ( p - ( 2 1-h ydrox y-3 ' -methacryloxypropoxy) phenylene) pr o p an e ,

sh o r te n ed to BIS-GMA, which i s d i l u t e d wi th o t h e r l e s s v i scous

monomers such as t r i e t h y l e n e g lycol d i m e th a c ry la te (TEGDMA). To

minimize po l ym er iza t io n shr ink age and to ins ure good c o l o r s t a b i l i t y

of th e compos i te , a minimum d i l u e n t co nt en t has been recommended

( Cowperthwaite e t a l . , 1979; Asmussen, 1981). Agains t t h e se

recommendations a r e da ta showing t h a t high BIS-GMA c on te n ts r e s u l t i n

composi te r e s i n s with r a t h e r low po ly me r iz a t io n degree (Ruyter &

Svendsen, 1978). There has a l s o been concern f o r some time among

ma nu fa c tu re rs about c e r t a i n problems a s s o c i a t e d wi th BIS-GMA. These

problems a re high water s o r p t i o n , high v i s c o s i t y , and low p u r i t y

(Glenn, 1979). However, dur ing the l a s t t en y e a r s , t h e r e has a l s o

been a t r e n d to r e p la c e BIS-GMA wi th o t h e r d i m e t h a c r y l a t e s . This

t r e n d i s most pronounced wi th r e s p e c t to the produc t io n o f the

s o - c a l l e d m i c r o f i l l e d compos ites in which ure tha ne d im e t h a c r y l a t e

r e s i n s a r e commonly used (Ruyter & S j0 v ik , 1981; Zinck e t a l . , 1982).

The f i l l e r system of most modern composi tes c o n s i s t s o f e i t h e r

q u a r t z , barium and s t r on t iu m g l a s s e s , or pyrogenic s i l i c a p a r t i c l e s

(Zinck e t a l . , 1982). The s i z e s of t h e s e p a r t i c l e s range w i t h in and

between pr od uc ts . Most q u a r t z and g l a s s f i l l e r s a r e 1-10 jum, whi le

the s i z e o f the pyrogenic s i l i c a , used in the m i c r o f i l l e d com pos i te s ,

12

O

a r e j u s t a few hundred A in s i z e (Ruyter & 0ysaed, 1982; Zinck e t a l . ,

1982). A d e f i c i e n c y of m i c r o f i l l e d m a t e r i a l s i s t h e i r r e l a t i v e l y low

f i l l e r c o n t e n t , which render s th e se m a t e r i a l s l e s s d im ens ion a l ly

s t a b l e in an environment with changes in tem pera ture and humidi ty .

T h e i r low f i l l e r co n te n t i s r e l a t e d to manufac tur ing problems with

in c o r p o r a t i n g high f r a c t i o n s of m i c r o f i l l e d p a r t i c l e s in to a monomer,

s i n c e th e c o n c e n t r a t i o n of m i c r o f i l l e d p a r t i c l e s d r a m a t i c a l l y changes

the v i s c o s i t y of the monomer ( J acobsen , 1981). To solve t h i s problem,

t h e manufac turer d i l u t e s the monomer wi th so l v e n t s such as ch loroform,

adds m i c r o f i l l e d p a r t i c l e s and evapora tes the s o l v e n t . By doing s o ,

r e s i n s co n ta in in g a high f r a c t i o n o f m i c r o f i l l e d p a r t i c l e s can be

produced. However, t h i s composite i s too v i scous to be handled

c l i n i c a l l y and t h e r e f o r e the manufac turer polymerizes t h i s f i l l e d

r e s i n and gr inds i t in to organic p a r t i c l e s co nt a in in g the in o r g a n i c

p a r t i c l e s . These composite p a r t i c l e s a re then mixed as a f i l l e r in to

the monomer and the product i s marketed as a m i c r o f i l l e d composi te ,

a l th ou gh the organi c f i l l e r p a r t i c l e s could be l a r g e r than

convent ional in or ga n ic f i l l e r p a r t i c l e s .

In a d d i t i o n to the developments of m i c r o f i l l e d compos i tes,

manufac turers have r e c e n t l y focused t h e i r e f f o r t s toward t h e

development of th e s o - c a l l e d hybr id or blend composi tes (Dennison,

1982). These m a t e r i a l s a r e made by admixing m ic r o - f in e and more

c o n v e n t i o n a l l y - s i z e d p a r t i c l e s i n to one r e s i n b a se , thereby making i t

p o s s i b l e to in c r e a s e the f i l l e r f r a c t i o n anothe r 10% by weight com

pa red to the more t r a d i t i o n a l composi tes (Dennison, 1982).

Even though the ma t r ix and the f i l l e r a re the main components o f

a den tal compos i te , p robably the most c r i t i c a l reg ion i s t h e i n t e r f a c e

13

between th e se two components (Bowen, 1972; Braden, 1978). To ach ieve

a bonding between mat r i x and f i l l e r , th e ino rg ani c f i l l e r p a r t i c l e s

a r e si l ane t r e a t e d , in most case s wi th / -m e th a c ry lo x y p ro p y l t r im e th o x y

s i l a n e , in or de r to form a chemical bond between mat r ix and f i l l e r s

(K o e l l i n g & Kolb, 1965; Plueddemann, 1970). During th e l a s t ten

y e a r s , a t te mp ts have a l s o been made to process f i l l e r s wi th such

topography t h a t they would be ab le to bond mechanic a l ly to the r e s i n

(Bowen & Reed, 1976a,b; Ehrnford , 1976).

In a d d i t i o n to th e se e f f o r t s to improve the phys ica l p r o p e r t i e s

of den ta l compos i tes , a t te m pt s to improve the handl ing p r o p e r t i e s have

a l s o been made. For example, a t r a n s i t i o n from co ld -c u re d r e s i n s

toward l i g h t - c u r e d r e s i n s has occur red dur ing the l a s t seve ra l y e a r s .

During t h i s p e r i o d , composite r e s i n systems based on a t e r t i a r y amine-

benzoyl peroxide a c t i v a t o r - i n i t i a t o r system were rep la ced by

compos i tes a c t i v a t e d by an u l t r a v i o l e t (UV) l i g h t source and i n i t i a t e d

by methyl e t h e r of benzoin. Due to concerns about the e f f e c t o f UV

l i g h t on ora l t i s s u e s as well as problems r e l a t e d to the

po lym er iza t ion depth of UV l i g h t - c u r e d r e s i n systems , v i s i b l e

l i g h t - c u r e d systems were in t r o d u c e d . In th e se v i s i b l e l i g h t - c u r e d

sys tems , camphoroquinone and N,N-dimethylaminoethyl m e th a c ry la te a r e

a c t i v a t e d by l i g h t wi th a wavelength o f 468 nm (Dar t & Nemcek, 1978).

Two advantages of t h i s system compared to the UV l i g h t system are a

lower r i s k f o r s o f t t i s s u e damages and g r e a t e r po ly me r i za t io n dept h ,

even though the po l ym er iza t ion depth i s not comple te ly s a t i s f a c t o r y

(Swar tz e t a l . , 1983). However, r i s k s of o c u l a r damage due to d i r e c t

and i n d i r e c t l i g h t exposure from the v i s i b l e l i g h t source seem t o

e x i s t (Antonson & Benedet to , 1984).

14

P r e s e n t Shortcomings Regarding Composites

Although composite r e s i n s have provided many advantages in r e s t o r a t i v e

t r e a t m e n t through the i n t r o d u c t i o n o f the ac id e tch techn ique and

improved composite sys tems , composi tes s t i l l a re not s u i t a b l e f o r

p o s t e r i o r r e s t o r a t i o n s except f o r Class I r e s t o r a t i o n s where e s t h e t i c s

i s impor tan t (Schro tenboer e t a l . , 1983). The main shor tcomings o f

p r e s e n t composite r e s i n a re inadequate wear r e s i s t a n c e , d i s c o l o r a t i o n

p o t e n t i a l , and t h e i r p o s s i b l e a s s o c i a t i o n with r e c u r r e n t c a r i e s

( L e i n f e l d e r e t a l . , 1980; Mjtfr, 1981). Of th e se drawbacks, the poor

wear r e s i s t a n c e as well as th e p o s s i b l e r e l a t i o n s h i p with r e c u r r e n t

c a r i e s have so f a r r e s t r i c t e d the usage o f compos ites to a r ea s sub

je c t e d to r e l a t i v e l y low loads and to l o c a t i o n s where i t i s easy t o

c l e a n the r e s t o r a t i o n and i t s sur rounding margins . Regarding

r e c u r r e n t c a r i e s , the evidence i s not co nc lu s iv e t h a t the composi tes

a v a i l a b l e today a re i n f e r i o r to amalgams (Mj0r, 1981). In a d d i t i o n to

th ese shor tcomings , t h e r e a re seve ra l qu es t io n s r e g a rd in g

b i o c o m p a t i b i l i t y which must be answered befo re one can s t a t e t h a t

composi tes a re as s a f e or s a f e r than denta l amalgams (S ta n l ey e t a l . ,

1979).

An improvement in wear r e s i s t a n c e of composi te r e s i n s has become

t h e most cha l l e n g in g re se a rc h problem in the e a r l y 198 0 ' s . In the

denta l l i t e r a t u r e , the wear mechanism of denta l composi tes has been

e x p la in e d as a r e s u l t o f exposure o f th e i no rg ani c f i l l e r by lo s s of

r e s i n m a t r ix . Subsequent ly , f i l l e r p a r t i c l e s a r e debonded and l o s t ,

and new r e s i n m a t r ix becomes exposed (Kusy & L e i n f e l d e r , 1977).

However, t h i s d e s c r i p t i o n i s too general to e x p la in the ba s i c f a i l u r e

mechanism. The wear process should be ana lyzed in g r e a t e r depth to

15

dete rmine whether t h e r e i s any s p e c i f i c type of wear mechanism which

i s dominant. The d i f f e r e n t types of wear processe s a r e as fo l lo w s

( Burwel1 , 1957):

1. Abrasive wear

2. Adhesive wear

3. Corros ive or chemical wear

4. Sur face f a t i g u e

Abras ive wear can f u r t h e r be d iv id ed in to e i t h e r two-body o r

th r ee -b o d y ty p e s . Each of th e s e types can occur in the ora l c a v i t y .

During s l i d i n g of a cusp over and in d i r e c t c o n t a c t with a r e s t o r a

t i o n , a two-body type of wear w i l l occur (Powers e t a l . , 1974).

However, i f food or any d e b r i s i s d i s p e rs e d between the cusp and t h e

r e s t o r a t i o n , a th r ee -bo dy type of a b r a s i v e wear e x i s t s (Prasad &

C a l v e r t , 1980). Except f o r a b ra s iv e wear , adhes ive wear can a l s o

o ccur under high p re s su re caus ing loca l c o n t a c t p o in ts o f both occ lud

ing cusp and r e s t o r a t i o n to fu se t o g e t h e r fol lowed by a t e a r i n g of t h e

s u r f a c e dur ing d is lo d g in g (Craig & Powers, 1976). The t h i r d ty pe ,

c a l l e d c o r r o s i v e or chemical wear , might a l s o be a c o n t r i b u t i n g

f a c t o r , as chemica ls could r e a c t and weaken m a t r i x , f i l l e r s , o r

f i l l e r - c o u p l i n g a g e n t , the reby f a c i l i t a t i n g wear of the composi te

(Bowen, 1972; Söderholm, 1979; Wu & McKinney, 1982). The f o u r t h type

of wear , induced by f a t i g u e , i s p robably a l s o an impor tan t p rocess t o

c o n s i d e r in the breakdown o f composi tes s in ce wear i s t ime-dependent

(Draughn, 1979; McCabe & Smith, 1981; McKinney & Wu, 1982).

Regarding t h e s e fo ur types o f wear , most r e s e a r c h e r s dea l i n g wi th

the wear o f den ta l compos ites have focused on a b r a s i v e wear (Craig &

Powers, 1976). The general f i n d i n g s from th e s e s t u d i e s have so f a r

16

been r a t h e r d i s cou ra g i ng as th e r e s u l t s have not c o r r e l a t e d well with

r e s u l t s from c l i n i c a l s t u d i e s (O'Neal & Eames, 1973; Powers e t a l . ,

1974).

To exp la in t he se d i s c r e p a n c i e s between l a b o r a t o r y s t u d i e s and

c l i n i c a l f i n d i n g s , an in c r e a s i n g number of r e s e a r c h e r s have sugges ted

t h a t i n t e r n a l s t r e s s e s as well as f a t i g u e could be the major

c o n t r i b u t i n g f a c t o r s in th e wear process (Draughn, 1979; McCabe &

Smith, 1981; McKinney & Wu, 1982). Another p o s s i b i l i t y could a l s o be

t h a t chemica ls in the ora l environment might r e a c t with any o f the

d i f f e r e n t components of a den ta l composi te (Bowen, 1972; Beech &

Brown, 1972; Söderholm & C a l v e r t , 1980; McKinney, 1984). However, no

u n i v e r s a l l y - a c c e p t e d theory has y e t been developed to ex pl a in the poor

wear r e s i s t a n c e of denta l compos i tes . From a v a i l a b l e c l i n i c a l d a t a ,

i t appears t h a t as the most w e a r - r e s i s t a n t composi tes a re t h e

m i c r o f i l l e d m a t e r i a l s (Meier & Lu tz , 1978; Jorgensen e t a l . , 1979;

Chr i s t ensen & C h r i s t e n s e n , 1982; Knudsen & Jo rg en sen , 1983). In

a d d i t i o n to th e improved wear r e s i s t a n c e of t h e s e m a t e r i a l s , the

m i c r o f i l l e d composi tes can be r e a d i l y po l i s he d wi th smoother s u r f a c e s

than any o t h e r composite (Dennison e t a l . , 1979).

In a d d i t i o n to the m i c r o f i l l e d compo s i tes , some s t ron t iu m g l a s s

f i l l e r - c o n t a i n i n g composi tes have a l s o shown some promising c l i n i c a l

r e s u l t s compared to q u a r t z - c o n t a i n i n g composi tes (Moffa & J e n k i n s ,

197 8a ,b ) . However, some c o n f l i c t i n g da ta a re a v a i l a b l e rega rd in g the

wear r e s i s t a n c e of the se s t r o n t i u m c o n ta in in g composi tes ( L e i n f e l d e r

e t a l . , 1982).

Regarding the f i l l e r - r i c h hybr id composi tes and t h e i r wear

r e s i s t a n c e , too few c l i n i c a l s t u d i e s a r e a v a i l a b l e to g ive conc lus i ve

17

in f o r m a t i o n . However, a r e c e n t s tudy i n d i c a t e s t h a t th e s e m a t e r i a l s

are not s u p e r i o r to o th e r composi tes (Ameye e t a l . , 1984).

Summing-Up

Although severa l e f f o r t s have been made to improve the wear r e s i s t a n c e

o f denta l compos i tes , no s u b s t a n t i a l b reak through can be i d e n t i f i e d

s ince the f i r s t commercial composi tes were in t ro du ced . As the coup

l i n g agent has remained r e l a t i v e l y unchanged, whi le o t h e r r e s i n - f i l l e r

components have been t r i e d , the weak l i n k could very well be t h e

f i l l e r - m a t r i x bond (Bowen, 1972; Braden, 1978). From t h i s viewpoin t

i t i s s u r p r i s i n g t h a t only a small number of r e s e a r c h e r s have s t u d i e d

t h i s v a r i a b l e . F o r t u n a t e l y , t h i s i n t e r f a c e region has been

i n v e s t i g a t e d in aerospace and naval r e sea rch programs, as

f i b e r - r e i n f o r c e d p l a s t i c s a r e very usefu l m a t e r i a l s in th es e

d i s c i p l i n e s . From the se s t u d i e s , i t has been shown t h a t water can

reduce the s t r e n g t h o f f i b e r r e i n f o r c e d r e s i n s s u b s t a n t i a l l y . Water

can a l s o a t t a c k g l a s s f i b e r s in a r e s i n m a t r i x , and f i b e r - m a t r i x

debonding as well as c rack formation might occur in t he se m a t e r i a l s

with t ime (Er ickson & Plueddemann, 1974). Based on t h i s i n fo rm a t io n ,

i t seemed rea son ab le to i n v e s t i g a t e th e e f f e c t o f a f i l l e r - m a t r i x

bonding on compressive s t r e n g t h of denta l compos i tes. S ince

p r o g r e s s i v e f i l l e r - m a t r i x debonding might be induced by e i t h e r water

o r thermal f l u c t u a t i o n s , th e se two v a r i a b l e s and t h e i r in f l u e n c e on

t h e f i l l e r - m a t r i x i n t e r f a c e were a l s o i n v e s t i g a t e d . A p ro g re s s iv e

f i l l e r - m a t r i x debonding could expla in why denta l composi tes f a i l due

t o wear a f t e r some t ime in s e r v i c e . Severa l l a b o r a t o r y s t u d i e s have

shown t h a t composi tes con ta in in g bonded f i l l e r s a re much more wear

18

r e s i s t a n t than th o se with unbonded f i l l e r s (Powers e t a l . , 1974,

Prasad & C a l v e r t , 1980). In a d d i t i o n to the p o s s i b l e e f f e c t s o f

debonding on th e wear p r o p e r t i e s , such a debonding could a l s o

in f lu e n c e thermal expansion as a m a t r ix wi th unbonded f i l l e r might

expand wi thout r e s t r a i n t in c o n t r a s t to a ma t r ix wi th bonded f i l l e r s .

Debonding would a l s o be ab le to in f lu e n c e the c o l o r of a compos i te , a s

s t a i n s could be accumulated in f i l l e r - m a t r i x gaps . These gaps would

in tu r n in f lu e n c e the opa c i t y of the composite due to a change i n

r e f r a c t i v e index a t th e f i l l e r - m a t r i x i n t e r f a c e .

19

OBJECTIVES

The p r i n c i p a l o b j e c t i v e s of the p r e s e n t s tudy were:

1. To i n v e s t i g a t e the in f l u e n c e o f f i l l e r f r a c t i o n as well as s i l a n e -

t r ea tm en t on the compressive s t r e n g t h of an exper imenta l composi te

m a t e r i a l .

2. To determine the e f f e c t o f water on the f i l l e r - m a t r i x i n t e r f a c e t o

e v a l u a t e whether f i l l e r - m a t r i x debonding might occur wi th time

in a composite s to re d in a wet environment .

3. To s tud y the water s o r p t i o n c a p a c i ty and d i f f u s i o n behav ior of

water in a r e s i n of s i m i l a r compos it ion as a commercia l ly

a v a i l a b l e denta l composi te r e s i n .

4. To s tudy e f f e c t s o f water on some commercial p r od uc ts .

5. To determine th e f i l l e r - l e a c h i n g behavior of commercial composite

r e s i n s over a s i x month p e r io d .

6. To i n v e s t i g a t e t he e f f e c t s o f f i l l e r f r a c t i o n s and f i l l e r t r e a t

ments on the thermal expansion c o e f f i c i e n t s o f den ta l compos i tes .

20

EXPERIMENTAL PROCEDURES AND RESULTS

I . RELATIONSHIP BETWEEN COMPRESSIVE YIELD STRENGTH AND FILLER

FRACTIONS OF PMMA COMPOSITES.

The o b j e c t i v e of t h i s i n v e s t i g a t i o n was to s tudy how volume f r a c t i o n

a s well as s u r f a c e t r e a tm e n t o f f i l l e r s i n f lu e n c e th e compressive

s t r e n g t h of den tal compos i tes .

M a t e r i a l s and Methods

Soda lime g l a s s beads , 4-44 jum in s i z e were used as f i l l e r s in t h e s e

exper ime nta l composi tes . In some compo s i tes , th e s e beads were p r e

t r e a t e d with s i l a n e . This t r e a tm e n t was done in a 250 ml s o l u t i o n o f

to lu ene co n ta in in g 2 v/o / - m e th ac ry lo xy pr opy l t r im eth oxy s i l a n e , i n

which 50 g f i l l e r was s t i r r e d f o r 24 h. The si l a n e - t r e a t e d f i l l e r was

then f i l t e r e d away from the s o l u t i o n and dr ie d a t 60°C f o r ano the r 24

h.

The exper imenta l composi te m a t e r i a l s were made by adding a

prede t e rmin ed volume f r a c t i o n f i l l e r (10 , 20, 30, 40, and 50 v /o) to a

20 ml j a r co n ta in in g some methyl m e th a c ry la te monomer. Af t e r t h e

t o t a l volume of f i l l e r had been added, th e j a r was comple te ly f i l l e d

with monomer and c l o s e d . In orde r to polymerize the monomer, 1 w/o

benzoyl peroxide and 0 .3 v /o N ,N -d im e th y l - p - to lu id in e were added to

the monomer befo re the f i l l e r was in c o r p o r a t e d . Af te r the j a r had

been s e a l e d , the m a t e r i a l was mixed in a machine by r o t a t i n g th e j a r 5

rpm f o r about 1 h. Following t h i s p r ocedure , the v iscous m a te r ia l was

t r a n s f e r r e d to a sy r in g e and i n j e c t e d i n t o g l a s s rods wi th an in ne r

21

d ia m e te r o f 4 .0 mm. A f te r s e t t i n g , th e m a t e r i a l s were h e a t - t r e a t e d a t

90°C f o r 12 h, c u t and ground to le ng th s of 8 .0 mm, and s t o r e d in a

d e s i c c a t o r a t room te m pe ra tu re . This technique was used to make

u n f i l l e d PMMA samples.

The 0.2% compressive y i e l d s t r e n g t h was dete rmined on t h r e e

samples from each group a t room tempera ture us ing an I n s t r o n t e s t i n g

machine and a c r os s -h ead speed of 1.27 mm/min. The s t r e n g t h v a lues

were analyzed by use o f l i n e a r r e g r e s s i o n a n a l y s i s assuming t h a t a

l i n e a r r e l a t i o n s h i p between s t r e n g t h and f i l l e r f r a c t i o n e x i s t e d .

A f t e r the compressive s t r e n g t h t e s t , the f r a c t u r e s u r f a c e s were

coa ted with 50 A of gold and s tu d i e d in a scanning e l e c t r o n microscope

a t lOkV. Spec ia l a t t e n t i o n was paid to i n v e s t i g a t e the morphology of

the f i l l e r - m a t r i x i n t e r f a c e s .

Re sul t s

A l i n e a r r e l a t i o n s h i p was found between compressive s t r e n g t h and

volume f r a c t i o n o f f i l l e r f o r 0

22

I I . DEGRADATION OF GLASS FILLER IN EXPERIMENTAL COMPOSITES.

The purpose of t h i s i n v e s t i g a t i o n was to e v a lu a te th e e f f e c t s o f w a te r

on th e s t a b i l i t y o f the f i l l e r - m a t r i x i n t e r f a c e in some exper imenta l

composi tes c on ta in in g soda- l ime g l a s s f i l l e r s .

M a t e r i a l s and Methods

Four experimenta l den tal compo s i tes , a l l c o n ta in in g about 40 v / o

f i l l e r , were prepared f o r s tu d y . Two o f th e se m a t e r i a l s used co ld -

cured polymethyl m e th a c ry la te (PMMA) as a ma t r i x m at e r i a l whi le t h e

o t h e r two m a t e r i a l s used a copolymer of b isphenol A g ly c id y l methacry

l a t e / t r i - e th y l e n e g lycol d im e t h a c r y l a t e (BIS-GMA/TEGDMA). The BIS -

GMA/TEGDMA composi tes conta ined 64 mol% BIS-GMA and 36 mol% TEGDMA and

were made to s im u la te a commercial ly a v a i l a b l e composi te . Th is

m a t e r i a l conta ined 1 w/o benzoyl peroxide and, in c o n t r a s t to th e PMMA

m a t e r i a l s , i t was hea t - c u red a t 60°C f o r 1 h, 75°C f o r 1 h , and 90°C

f o r 4 h , and subseque nt l y i t was s lowly cooled to room tempe ra ture .

Of th e two types of composi tes us ing the same mat r ix sys tem, one

o f th e f i l l e r s had been s i l a n e - t r e a t e d and th e o t h e r f i l l e r was

u n t r e a t e d . The f i l l e r s used in a l l th e s e composi tes were 4-44 jum

so da - l im e g l a s s beads. S i l a n e t r e a t m e n t and sample p r e p a r a t i o n were

performed as d e sc r ib ed in Paper I .

Groups of t h r e e samples of each m a te r i a l were s t o r e d in 20 ml

d i s t i l l e d water in se a le d a c r y l i c b o t t l e s a t 60°C. These samples were

f r a c t u r e d by a d iame t ra l compressive t e s t a f t e r 16, 32, and 180 days

i n w a te r . A f t e r f r a c t u r e t h e samples were coa ted wi th gold in

p r e p a r a t i o n f o r scanning e l e c t r o n microscopy.

23

Another s e r i e s was des igned to determine changes in th e pH and

the sodium con te n t o f the s t o r a g e water wi th t ime . An 80-day s t o r a g e

p e r i o d was a r b i t r a r i l y s e l e c t e d and during t h i s t ime th e pH was

r e g u l a r l y determined with a pH meter and a g l a s s e l e c t r o d e . The

sodium c o n c e n t r a t i o n of the s t o r a g e w ate r was de te rmined a t the end of

t h i s 80-day per iod by atomic abs o rp t io n sp ec t ro p h o to m e t r y , us ing a

hol low cathode lamp, an a i r - a c e t y l e n e f lame, a 0 .7 nm wide s l i t , and a

wavelength of 589.0 nm.

Three dry samples of th e BIS-GMA/TEGDMA composi te wi th s i l a n e -

t r e a t e d g l a s s beads were cyc led in compression with 4400 loads f o r one?

hour us ing a s t r e s s leve l ranging from 0 to 15 N/mm . Three more

samples of the same m a te r i a l were cyc led f o r one hour wi th the same2

f requency , but with a s t r e s s i n t e r v a l of 0-45 N/mm . These specimens,

c l a s s i f i e d as low- and h i g h - s t r e s s e d samples , r e s p e c t i v e l y , were

t h e r e a f t e r immersed in 20 ml d i s t i l l e d water and kept a t 60°C f o r 80

day s . The s to ra g e water o f th e s e samples was a l s o ana lyzed r ega rd ing

pH and sodium c o n c e n t r a t i o n .

R esul t s

Degradat ion of th e f i l l e r , e x h i b i t e d as a p e e l in g of th e f i l l e r

s u r f a c e , could be seen on some o f the u n t r e a t e d g l a s s beads i n s i d e the

BIS-GMA/TEGDMA mat r ix a f t e r 16 days in w a te r . This f i l l e r d e g r a d a t i o n

was lo c a t e d c lo se to th e o u t e r s u r f a c e o f th e samples.

Af te r 32 days in w a te r , s u r f a c e changes of the f i l l e r s were seen

i n a l l composi tes wi th u n t r e a t e d f i l l e r s . The d e f e c t s seen in the

BIS-GMA/TEGDMA compos ites were more pronounced than those in th e PMMA

compos i tes .

24

The PMMA compos ites c o n ta in in g u n t r e a t e d f i l l e r s s u f f e r e d from

severe f i l l e r - s u r f a c e deg ra da t io n a f t e r the s to r a g e t ime was ex tended

t o 180 days. This degr ada t io n reached a depth o f about 0 .9 juni, and

the degraded g l a s s l a y e r had a swollen appearance . In t h e

BIS-GMA/TEGDMA composi tes c o n ta in in g u n t r e a t e d f i l l e r s , th e g l a s s

s u r f a c e had a swollen appea rance , too .

No d i r e c t s i g n s of f i l l e r - m a t r i x debonding were seen in compos

i t e s c o n ta in in g s i l a n e - t r e a t e d f i l l e r s . However, l e s s e l e c t r o n dense

a r e a s and p r e c i p i t a t e - l i k e format ions could be d e te c te d on some of

the se beads .

The pH o f w ater in which the d i f f e r e n t composi tes had been

s t o r e d inc re ase d wi th t ime . This i n c r e a s e was most pronounced f o r

composi tes us ing PMMA and u n t r e a t e d f i l l e r s , reaching a pH of 9 .5

a f t e r 80 days in w a te r . I t was a l s o demons tra ted t h a t th e pH

inc re ase d when the f i l l e r had been s i l a n e - t r e a t e d , but to a l e s s e r

e x t e n t . The i n c r e a s e in pH was remarkably small f o r the

BIS-GMA/TEGDMA compos ites con ta in in g s i l a n e - t r e a t e d f i l l e r , but became

l a r g e r a f t e r a s e r i e s of c y c l i c s t r e s s e s .

The sodium ion c o n c e n t r a t i o n of the d i s t i l l e d water in which t h e

compos i tes were s t o r e d showed an in c r e a s e from 0 . 1 - 0 . 2 ppm to 5-9.5

ppm a f t e r 80 days. No d i r e c t r e l a t i o n s h i p s could be seen between t h e

s i l a n e - t r e a t e d and u n t r e a t e d f i l l e r compos i tes .

25

I I I . WATER SORPTION IN A BIS-GMA/TEGDMA RESIN.

The p r e s e n t s tudy was under taken to determine some water s o r p t i o n

p r o p e r t i e s of a copolymer used as a m a t r i x in a dent a l compos i te . The

p r o p e r t i e s i n v e s t i g a t e d were l i n e a r s w e l l i n g , water s o r p t i o n c a p a c i t y ,

l e a c h i n g , and d i f f u s i o n c o e f f i c i e n t s .

M a te r ia l s and Methods

A combinat ion of BIS-GMA and TEGDMA r e s i n s were mixed and polymer ized

a s de sc r ib ed in Paper I I . Before p o ly m e r i z a t i o n , th e monomer was c a s t

in to a g l a s s rod approximate ly 10 cm long and wi th an i n n e r d ia m e te r

o f 21 mm whereupon the rod was sea led from both ends. A f t e r po lymer i

z a t i o n , f i v e d i s c s , each 0 .8 mm t h i c k , were c u t and smoothed by

p o l i s h i n g the c u t s u r f a c e s with sandpaper (250, 400, and 600 g r i t ) .

The samples were weighed and s to re d in a d e s i c c a t o r u n t i l s t a b l e

we ights had been ga ined . The d iameters of t he d i s c s were measured

with a measuring microscope and the t h ic k n e s s of the d i s c s were

measured wi th a micrometer .

Af te r dehydra t ion the d i s c s were i n d i v i d u a l l y s t o r e d in P e t r i

d i s h e s co nt a in in g 10 ml d i s t i l l e d water a t e i t h e r 60°C or 37°C depend

ing on the scheduled t r e a t m e n t . A f te r 15 and 30 min of s to r a g e i n

w a t e r , th e samples were weighed, whereupon th e weight gain was mea

sured every h a l f - h o u r dur ing the next 1.5 hours , and then every hour

d ur in g t h e fo l low ing 6 hours . Su bse quen t ly , th e weight ga in was

recorded every 12th hour u n t i l a s t a b l e va lue was reached . From

measurements of t h e d i ameters of th e samples a f t e r s a t u r a t i o n and

cooling to room tempera ture in w a te r , th e l i n e a r expansion caused by

26

w a t e r s o r p t i o n could be de te rmined. The d e so rp t i o n process was

s tu d i ed by s t o r i n g the w a t e r - s a t u r a t e d d i s c s in a d e s i c c a t o r a t e i t h e r

60°C or 37°C, depending upon the d es i gna te d t r e a t m e n t , and th e weight

loss was determined accord ing to the same time schedule as used du r ing

t h e s o r p t i o n s t u d i e s .

The d i f f u s i o n c o e f f i c i e n t s of water i n to the polymer d i s c s were

de te rmined by p l o t t i n g the M^/M^ r a t i o as a fu n c t i o n of t h e square

roo t o f t ime where was the weight gain a f t e r a t ime , t , and M^was

the f i n a l weight ga in . During d e s o r p t i o n , a s i m i l a r curve was p l o t

t e d , but in t h i s case was th e weight lo ss a f t e r a t i m e , t , and

was the f i n a l weight los s a f t e r deh ydr a t i on .

By measuring the changes in weight wi th t ime and p l o t t i n g t h e s e

r e s u l t s a t l e a s t f o r M̂ /M̂ 0 . 5 , a s t r a i g h t l i n e could be drawn f o r

the i n v e s t i g a t e d m a t e r i a l s . The method of l e a s t squares was used t o

de te rm ine the equa t io n f o r t h i s l i n e . This equa t ion pe rm i t t ed an

e s t i m a t i o n of the time i t took to reach M./M = 0 .5 . Knowing t h a tt oo

t i m e , t i9 the d i f f u s i o n c o e f f i c i e n t s could be c a l c u l a t e d from the 2

r e l a t i o n s h i p ,

D = 0 .049/ ( t j_ /L 2 )2

2 -1where D = d i f f u s i o n c o e f f i c i e n t (cm s ) , and L = t h ic k n e s s of d i s c

(cm) (Crank, 1957). The d i f f u s i o n c o e f f i c i e n t o f the polymer was

determined by tak in g the mean va lue of the s o r p t i o n and d e s o r p t i o n

c o e f f i c i e n t s as t h e " t ru e" d i f f u s i o n c o e f f i c i e n t (Crank 1957).

27

Results

The weight gain occur r in g dur ing the f i r s t s o r p t i o n a t 60°C was 3 .3 9

w/o. The weight gain was 3 .68 w/o dur ing the second cyc l e a t th e same

te m pera tu re . This in c re a s e in water s o r p t i o n c a p a c i t y dur ing t h e

second c y c le a t 60°C was s t a t i s t i c a l l y s i g n i f i c a n t acco rd ing to the

S t u d e n t ' s t - t e s t (p < 0 . 0 0 5 ) .

During the f i r s t s o r p t i o n - d e s o r p t i o n cyc le a t 60°C, a 0.31%

d e c r e a s e in weight oc cu r r ed . During th e fo l l owing s o r p t i o n - d e s o r p t i o n

cyc le a t 37°C, the samples i n c reased in weight 0.07%, which l a t e r was

l o s t dur ing the second cy c l e a t 60°C. The l i n e a r expans ion caused by

water s o r p t i o n was 1.07-1.08% and did no t change s i g n i f i c a n t l y between

t h e t h r e e c y c l e s . During th e f i r s t cyc le a t 60°C, th e de so rp t io n

d i f f u s i o n c o e f f i c i e n t was h ig he r than the s o r p t i o n d i f f u s i o n c o e f f i

c i e n t , whi le the r e v e r s e o rde r p r e v a i l e d dur ing th e second s o r p t i o n -

de so rp t io n cyc le a t 60°C ( p < 0 . 0 5 ) . At 37°C, no s t a t i s t i c a l l y

s i g n i f i c a n t d i f f e r e n c e was found ( p > 0 . 0 5 ) between the s o r p t i o n and

the d e s o rp t i o n d i f f u s i o n c o e f f i c i e n t s .

IV. LEACHING OF FILLERS IN DENTAL COMPOSITES

The aim of t h i s s tudy was to i n v e s t i g a t e the lea ching r a t e of s i l i c o n

from f o u r commercial den ta l composi tes s t o r e d in d i s t i l l e d w a te r , to

e v a lu a te whether h y d r o l y t i c degr ad a t i on of f i l l e r s in commercial

p ro duc ts could be d e te c te d and r e l a t e d to t h e leakage o f g l a s s -

modifying elements such as barium and s t r o n t i u m .

28

M ate r ia l s and Methods

Four com m erc i a l l y - av a i l a b l e dental compos ites (Concise , P r o f i l e ,

F o t o f i l ® , and S i l a r ® ) which conta ined d i f f e r e n t f i l l e r composi t ions

were s e l e c t e d f o r s tu dy .

The f i l l e r f r a c t i o n s of the d i f f e r e n t composi tes were de termined

by weight d e te rm in a t io n of samples befo re and a f t e r the m at r ix had

been burned away a t 575°C (ISO Standard 4049, 1978). The v a r io u s

f i l l e r s were analyzed by o p t i c a l emiss ion spec t rog raphy us ing photo

graphic reco rd ing fol lowed by i d e n t i f i c a t i o n of elements based on

v i s u a l comparison with s ta n d a rd s .

X-ray d i f f r a c t i o n s p e c t r a were taken of one sample of each

compos i te to e v a l u a t e whether th e f i l l e r was c r y s t a l l i n e or amorphous.

For the leaching d e te rm in a t io n s i x c i r c u l a r d i s c s , 15.0 mm i n

d ia m e te r and 0.85 mm t h i c k , were made of th e f o u r composite produc ts .

A f te r p o ly m e r iz a t i o n , one s i d e of each sample was s l i g h t l y roughened

by sandpaper to a s s u re exposure of f i l l e r p a r t i c l e s on t h a t s i d e . All

s ix samples of each composite m a te r ia l were p laced in a p l a s t i c b o t t l e

c o n t a i n i n g 10 ml d i s t i l l e d water and s t o r e d in an oven a t 60°C. A f te r

30 days , the samples were t r a n s f e r r e d to ano ther b o t t l e co n ta in in g 10

ml d i s t i l l e d w a te r whereupon ano the r 30 day per iod o f s t o rag e

f o l l o w e d .

A f te r the t o t a l 60 day p e r i o d , the water samples f o r the two 3 0

day i n t e r v a l s were ana lyzed by use of plasma spec t rosc opy to dete rmine

the c o n c e n t r a t i o n s of Al , B, Ba, Mg, Sb, Sr , T i , and S i .

29

Results

Except f o r the m i c r o f i l l e d r e s i n , each composite conta ined about 0 .6

volume f r a c t i o n f i l l e r . The m i c r o f i l l e d composi te con ta ine d only

about 0 .3 volume f r a c t i o n f i l l e r . The o p t i c a l emission sp ec t r o g ra p h y

a n a l y s i s showed t h a t Concise® and Si 1 ar® mainly conta ined S i , whi le(ß)P r o f i l e con ta ine d S i , Sr and Al. The main c o n s t i t u e n t s o f the f i l l e r

o f Fotof i 1® were S i , Ba and Al.

X-ray d i f f r a c t i o n a n a l y s i s showed t h a t a l l m a t e r i a l s , e x c e p t®

Concise , were amorphous in s t r u c t u r e . The c r y s t a l l i n e s t r u c t u r e of

Concise was i d e n t i c a l to / 3 -q u a r tz .

The plasma s p e c t r o s c o p ic a n a l y s i s showed t h a t P ro f i l e ® re l ea s e d Sr

and B, whi l e Fotofi 1 leaked Ba and B. All m a t e r i a l s l eaked S i , wi th® ®

the leakage most pronounced f o r P r o f i l e and F o to f i l dur ing the f i r s t

30 days.

Compared to the f i r s t 30-day p e r i o d , the leakage of Sr , B, and Ba

dec rea se d dur ing th e second 30-day p e r i o d , as d id the leakage o f S i .

An excep t ion was the m i c r o f i l l e d compos i te , which showed a lmost tw ice

a s high leakage o f Si dur ing the second per iod .

V. HYDROLYTIC DEGRADATION OF DENTAL COMPOSITES

This s tud y was under taken to e v a l u a t e t he leaching rpte o f s i l i c o n ,

s t ro n t ium and barium from four composite r e s i n s during a s i x month

p e r i o d . Special a t t e n t i o n was paid to d e t e c t any s ig ns of morphologi

cal changes which could i n d i c a t e ongoing h y d r o l y t i c p rocesse s a t t h e

f i l l e r - m a t r i x i n t e r f a c e s .

30

M a t e r i a l s and Methods

® ® ®Three compos i tes , P - 10 , P r o f i l e , and Den-Mat Class II , each

a d v e r t i s e d f o r use in p o s t e r i o r r e s t o r a t i o n s , and one m i c r o f i l l e d®

r e s i n , I s o p a s t , were i n v e s t i g a t e d .

Five samples of each m a t e r i a l , 15.0 mm long and 4 .0 mm in diame

t e r , were i n d i v i d u a l l y s t o r e d in 7 ml d i s t i l l e d water a t 60°C. A f t e r

t h i r t y days , the samples were t r a n s f e r r e d to o t h e r v i a l s cont a in in g 7

ml d i s t i l l e d w a te r whi le th e p re v io u s ly used s t o r a g e w a te r was kept

f o r f u t u r e a n a l y s i s . This process was repea t ed s i x t imes .

Five samples o f each composi te were ground f l a t and coa ted with

go ld. Analysi s o f the f i l l e r compos i t ion was performed us ing an

e n e r g y - d i s p e r s i v e x - ra y a n a ly z e r op era t ed a t an a c c e l e r a t i n g v o l ta ge

of 15 kV. Af te r i d e n t i f i c a t i o n of the p r i n c i p a l e leme nts , the compo

s i t i o n s were determined by use o f a computer program ab le to perform a

q u a n t i t a t i v e a n a l y s i s .

Based on th e f i l l e r compos i t ions determined by en ergy- d i s p e r s i v e

x - r ay a n a l y s i s , th e s t o r a g e water was ana lyzed f o r c o n c e n t r a t i o n s of

s i l i c o n , s t r o n t i u m , and barium. These atomic a b s o rp t io n s p e c t r o p h o to -

m e t r i c ana ly se s were performed by use of hollow cathode lamps and a

n i t r o u s o x id e - a c e ty le n e f lame. S i l i c o n , s t r o n t i u m , and barium were

ana lyzed a t wavelengths of 251.6 nm, 460.7 nm, and 553.6 nm, r e s p e c

t i v e l y , us ing a s l i t s e t t i n g o f 0 .2 nm. Analys is o f va r ia nc e was p e r

formed on the atomic ab s o r p t io n da ta c o l l e c t e d over th e six-month

p e r i o d .

Except f o r the a n a l y t i c a l p rocedu re s , l i g h t microscopy and

scanning e l e c t r o n microscopy were used to i n v e s t i g a t e the morphology

of the i n v e s t i g a t e d m a t e r i a l s both before and a f t e r s t o ra g e in w a te r .

31

Results

®The en e rg y - d i s p e r s i ve x- r ay a n a l y s i s showed t h a t the f i l l e r o f P-10

and I sopas t® mainly conta ined S i , whi le Pr o f i le ® conta ined aluminum,®

s i l i c o n , and s t r o n t i u m . The f i l l e r composi t ion of Den-Mat Class I I

was s i m i l a r to t h a t o f P r o f i l e except f o r a small barium co n te n t and

a lower s t ro n t iu m c o n c e n t r a t i o n .

The atomic ab so rp t i o n s p e c t r opho to m e t r ic a n a l y s i s r ev ea le d t h a t®

a l l m a t e r i a l s were leached of s i l i c o n . Except f o r s i l i c o n , P r o f i l e®

a l so r e l e a s e d s t r on t ium whi le the Den-Mat Class II composite l o s t

bo th s t r o n t i u m and barium. A c e r t a i n p r o p o r t i o n a l i t y e x i s t e d between

s i l i c o n , s t r o n t i u m , and barium. Regarding the lea ching p a t t e r n of t h e

d i f f e r e n t m a t e r i a l s , an i r r e g u l a r change with t ime was found except®

f o r I s o p a s t . The g r e a t e s t and second g r e a t e s t l each ing r a t e f o r

s i l i c o n were demonst rated by P ro f i l e ® and Isopast® , r e s p e c t i v e l y ,

while P-10 showed the lowest l each ing r a t e ( p < 0 . 0 5 ) .

Scanning e l e c t r o n microscopy showed t h a t c racks c lo se to or a t® ® ®

the s u r f a c e s could be seen on P-10 , P r o f i l e and Den-Mat Class I I

samples s to re d in w a te r . Of th e se t h r e e m a t e r i a l s , a l a r g e number of® ®

cracks was observed in P r o f i l e and Den-Mat Class II specimens b u t® ®

few cracks were found in th e P-10 specimens. I s o p a s t d id not show

any d e t e c t a b l e c r a c k s . The d e te c te d c racks were lo ca t ed a t t h e

m a t r i x - f i l 1er i n t e r f a c i a l zone and tended to propagate away from th ese

regions a t sharp f i l l e r co rn er s to o t h e r m a t r i x - f i l l e r i n t e r f a c i a l

a r e a s .

32

VI. INFLUENCE OF SILANE TREATMENT AND FILLER FRACTION ON THERMAL

EXPANSION OF COMPOSITE RESINS.

The o b j e c t i v e s of t h i s i n v e s t i g a t i o n were to s tudy the e f f e c t o f

f i l l e r f r a c t i o n on the c o e f f i c i e n t o f thermal expans ion and to

e v a lu a te the i n f lu e n c e of t he f i l l e r - m a t r i x bond q u a l i t y on t h e

c o e f f i c i e n t o f thermal expansion .

M a te r ia l s and Methods.

Experimental compos i tes , c o n ta in in g 0 . 2 , 0 .4 or 0 .6 volume f r a c t i o n o f

e i t h e r s i l a n e - t r e a t e d or u n t r e a t e d soda- l ime g l a s s beads were prepared

f o r s t udy . The mat r ix was t r i e t h y l e n e g lycol d im e th a c ry la te (TE6DMA),

t o which 0.15 w/o benzoyl peroxide and 0 .2 v/o N,N-dimethyl-p-

t o l u i d e n e were added to a c t i v a t e p o ly m er iz a t i on . In a d d i t i o n to t h e s e

c o m pos i t e s , pure r e s i n samples were a l s o made. These m a t e r i a l s were

placed i n t o g l a s s tubes 4 .0 mm in d i a m e te r , co l d -cu red and cu t i n t o

5 0 .8 mm long samples . A f t e r c u r i n g , t h e samples were s t o r e d a t 65°C

f o r 12 h.

The c o e f f i c i e n t o f thermal expansion of each sample was d e t e r

mined twice in a d i l a t o m e t e r . The tempera ture range and he a t i n g r a t e

f o r th e se measurements were 25°C to 90°C and 4°C/min, r e s p e c t i v e l y .

A l i n e a r r e g r e s s i o n a n a l y s i s was performed on c o e f f i c i e n t of

thermal expansion versus f i l l e r f r a c t i o n d a t a .

To e s t i m a te p o s s i b l e s t r e s s l e v e l s in a composi te a t room temper

a t u r e , a s i m p l i f i e d model o f a composi te co n ta in in g s p h e r ic a l f i l l e r s

was in t r od uced . Based on s t r e s s e s in a t h i c k - w a l l e d s p h e r ic a l sh e l l

caused by an i n t e r n a l p r e s s u r e , the h i g h e s t and lowest s t r e s s l e v e l s

33

which might occur in the i n v e s t i g a t e d composite could be es t imated

with t h i s model. The e f f e c t o f f i l l e r f r a c t i o n on th e se s t r e s s e s

could a l s o be v i s u a l i z e d from t h i s model by o rd in a ry c a l c u l a t i o n s .

R esu l t s

The l i n e a r r e g r e s s i o n a n a l y s i s showed t h a t the c o e f f i c i e n t o f thermal

expans ion of the i n v e s t i g a t e d m a t e r i a l s fol lows a l i n e a r r e l a t i o n s h i p

with f i l l e r f r a c t i o n ( p < 0 . 0 5 ) . Regarding th e d i f f e r e n t f i l l e r

t r e a t m e n t s , no d i f f e r e n c e s could be d e t e c t e d in thermal expansion f o r

a s p e c i f i c f i l l e r f r a c t i o n . However, thermal c y c l in g seemed to cause

a dec rease in the c o e f f i c i e n t o f thermal expansion d e t e c t e d during the

second c y c l e .

Combining the exper imental r e s u l t s with the s i m p l i f i e d model of a

composi te p re sen t ed in t h i s s t u d y , i t was shown t h a t f i l l e r p a r t i c l e s

a re surrounded by hoop s t r e s s e s induced dur ing po ly m er i za t i o n . These

s t r e s s e s a r e compressive in a r a d i a l d i r e c t i o n a t the f i 11e r - m a t r i x

i n t e r f a c e , while they a r e in te n s io n in a t a n g e n t i a l d i r e c t i o n .

These s t r e s s e s decrea se wi th d i s t a n c e from the f i 11e r - m a t r i x i n t e r f a

c ia l zone and become zero in a ra d ia l d i r e c t i o n a t the s u r f a c e of t h e

proposed composite model. Residual t e n s i l e s t r e s s e s remain in a

t a n g e n t i a l to the f i l l e r s u r f a c e . The magnitude of th ese s t r e s s e s a r e

dependent on f i l l e r f r a c t i o n . Low f i l l e r f r a c t i o n s g ive high compres

s ive s t r e s s e s and r e l a t i v e l y low t a n g e n t i a l t e n s i l e s t r e s s e s . Higher

f i l l e r f r a c t i o n s reduce the compressive r a d i a l s t r e s s e s but in c re as e

the t a n g e n t i a l t e n s i l e s t r e s s e s .

34

DISCUSSION

Paper I shows t h a t a good f i l l e r - m a t r i x bond i s re q u i r e d when a d e n ta l

compos i te i s made. Lack o f such a bond w i l l weaken r a t h e r than

s t re n g th e n a composite m a te r i a l i f more f i l l e r i s added, a t l e a s t when

a s p h e r i c a l f i l l e r i s used. I f th e f i l l e r i s well bonded to the

mat r ix me chanica l ly or c he m ic a l l y , an in c re a se in compressive s t r e n g t h

can be expected wi th an i n c r e a s e in f i l l e r f r a c t i o n ( I sh a i & Cohen,

1968; Pra sad , 1977).

Through the y e a r s the genera l t r e n d has been to use as high a

c o u p l e d - f i l l e r f r a c t i o n as p o s s i b l e (Dogon e t a l . , 1982). Attempts

have a l s o been made to improve the mechanical r e t e n t i o n of f i l l e r

w i t h i n the mat r ix (Bowen & Reed, 1976a,b; Ehrnford, 1976, 1981, 1983).

The b e n e f i t s o f t h i s in c r e a s e in both f i l l e r f r a c t i o n and f i l l e r

bonding have r e s u l t e d in h i ghe r compressive s t r e n g t h and modulus of

e l a s t i c i t y , and lower po lym er iza t ion s h r in k a g e , thermal expans ion , and

w a t e r - s o r p t i o n / w a t e r - s w e l 1ing o f the denta l composite (Bowen & Rodr i

guez, 1962; Bowen, 1964).

Even though l a r g e r f i l l e r f r a c t i o n s and b e t t e r f i l l e r - m a t r i x

bonding improve th e p r o p e r t i e s of compos i tes , i t should be noted t h a t

the in d iv id ua l composite components r e t a i n t h e i r o r i g i n a l p r o p e r t i e s

(H o l l i d a y & Mann, 1966). Accord ingly , the i no rg ani c f i l l e r does not

improve the p r o p e r t i e s of t he r e s i n per s e , but merely a c t s to reduce

t h e u n d e s i r a b le p r o p e r t i e s of the polymer. Due to g r e a t d i f f e r e n c e s

in some phys ica l p r o p e r t i e s of the m at r i x and th e f i l l e r , one can

s u s p e c t t h a t hygrothermal changes as well as po lym er iza t ion shr inkage

could induce r e l a t i v e l y l a r g e s t r e s s e s a t the f i l l e r - m a t r i x i n t e r f a c e s

35

(Hahn & Kim, 1978; H a r r i s , 1978). These s t r e s s e s , as well as chemi

c a ls a s s o c i a t e d wi th the ora l envi ronment , might cause f i l l e r debond

ing from the m at r ix with t ime (Söderholm & C a l v e r t , 1980). Thus, the

d e t e r i o r a t i o n of the f i l l e r - m a t r i x bond with t ime wi l l determine t h e

long- te r m p r o p e r t i e s of a composite r e s t o r a t i o n . The e f f e c t of

f i l l e r - m a t r i x debonding was a l s o i n d i r e c t l y demonst rated in Paper I .

In t h i s s tudy i t i s shown t h a t the compressive s t r e n g t h of composites

con ta in in g 0.5 volume f r a c t i o n of unbonded f i l l e r was only o n e - h a l f

t h e s t r e n g t h o f composi tes wi th bonded f i l l e r . In a d d i t i o n to the

s i g n i f i c a n c e of good m a t r i x - f i 11er bonding on compressive s t r e n g t h ,

i t s e f f e c t on th e wear p r o p e r t i e s of composi tes has a l s o been shown

(Powers e t a l . , 1974; Prasad & C a l v e r t , 1980). In one s tudy on wear ,

i t was shown t h a t composi tes c o n ta in in g unbonded f i l l e r wore as f a s t

o r f a s t e r than the pure mat r ix m a t e r i a l , while composi tes c o n t a i n i n g

bonded f i l l e r s were much more w e a r - r e s i s t a n t (Pr asad , 1977).

Accord ingly, the poor c l i n i c a l long- te rm wear r e s i s t a n c e of d e n ta l

compos i tes could be a r e s u l t o f m a t r i x - f i l l e r debonding o ccur r in g in

the ora l c a v i t y wi th time (Bowen, 1972; Braden, 1978).

To i n v e s t i g a t e the proposed f a i l u r e mechanism, s ev e ra l f a c t o r s

such as po l ym er iza t io n s h r in k a g e , hygrothermal i n c o m p a t i b i l i t y , and

chemical p r ocesses must be con s i de red . Before any th e o ry can be

proposed regard in g such a debonding mechanism, i t i s impor tan t t o

c h a r a c t e r i z e th e f i l l e r - m a t r i x i n t e r f a c e . The s u r f a c e between the

r e s i n and the f i l l e r can be i d e n t i f i e d as the t r u e f i l l e r - m a t r i x

i n t e r f a c e . However, t h i s su r f a c e i s d i f f i c u l t to s tudy due to i t s

i n f i n i t e s i m a l l y small volume. As most adhes ive bond f a i l u r e s

d e s c r i b e d as i n t e r f a c e debonding occur in a reg ion very c l o s e to but

36

n o t e x a c t l y a t the t r u e i n t e r f a c e , the term f i l l e r - m a t r i x i n t e r f a c e is

in r e a l i t y synonymous with the i n t e r f a c e reg i on . This reg ion in c lu d e s

a t h i n l a y e r of th e f i l l e r a t the f i l l e r s u r f a c e , the coupl ing a gen t ,

and a t h i n l a y e r of the m at r ix a t the coupl ing age n t -m a t r ix i n t e r f a c e .

Each o f th e se components have t h e i r own s p e c i f i c phys ica l and chemical

p r o p e r t i e s (Kwei, 1965).

Regarding the s i l a n e l a y e r and i t s a b i l i t y to bond to the f i l l e r

s u r f a c e , most r e s e a r c h e r s c u r r e n t l y agree t h a t i t s f i l l e r bonding

a b i l i t y i s e i t h e r o f a co v a l en t or p o l a r na tu re (Koel l ing & Kolb,

1965; Plueddemann, 1970). Whether th e s e two types of bonds, e s p e c i a l

ly the hydrogen bond, a re s t a b l e in an aqueous environment has been an

open ques t io n f o r severa l y e a r s in l i t e r a t u r e de a l i n g wi th r e i n f o r c e d

p l a s t i c s (Er ickson & Plueddemann, 1974). The r e s u l t s p r esen t ed i n

Paper I I i n d i c a t e t h a t the s i l a n e coupl ing agent used in t h i s s tudy

was s t a b l e a g a i n s t water a t t a c k over the d u r a t i o n of the s t u d y .

However, the weak p o in t o f th e i n t e r f a c e region was the su r f a c e o f the

f i l l e r p a r t i c l e s , which showed s ig ns of h y d r o l y t i c d e g r a d a t i o n ,

e s p e c i a l l y when th e f i l l e r was not s i l a n e t r e a t e d . This degrada t ion

i s exp la ine d by assuming t h a t water reaching the f i l l e r - m a t r i x

i n t e r f a c e s leach sodium ions from th e g l a s s s u r f a c e (C h a r le s , 1958).

As a r e s u l t o f t h i s l each ing p r o c e s s , the s i l i c a network becomes

n e g a t i v e l y charged due to lo s s of p o s i t i v e sodium io ns . To compensate

fo r t h i s charge imbalance, hydrogen ions of the wa ter sur rounding t h e

g l a s s s u r f a c e a r e a t t r a c t e d in to the s i l i c a network. Through t h i s

p ro c e s s , hydroxyl ions of th e water accumulate o u t s i d e the g l a s s

s u r f a c e , the reby i n c r e a s i n g the pH in t h a t reg io n . At a c e r t a i n pH

l e v e l , th e se hydroxyl ions a t t a c k th e s i lo x an e bonds inducing a

37

h y d r o l y t i c d e gr ad a t io n of th e f i l l e r s u r f a c e . These r e a c t i o n s can be

de scr ibe d as fo l lows (C h a r le s , 1958):

- S i - 0 ' Na+ + H20 — -Si-OH + Na+0H" (1)

- S i - 0 - S i -OH + OH' — - S i - 0 ' + HO-Si-OH (2)

Thus, the r e a c t i o n s between the hydroxyl ions and the s i l i c a

network r e s u l t in a breakdown of the s i l i c a s t r u c t u r e . As one of

t h e s e r e a c t i o n produc ts i s - S i - 0 " , water w i l l cont inue to r e a c t wi th

the s i l i c a network accord ing to the r e a c t i o n

- S i - 0 ’ + H20 — -Si-OH + OH’ (3)

Due to proton t r a n s f e r , new hydroxyl ions a r e formed which

c o n t i n u e to r e a c t wi th the s i l i c a s t r u c t u r e , and h y d r o l y t i c degrada

t i o n the re by becomes a u t o c a t a l y t i c (C h a r l e s , 1958).

The r e s u l t s p r e se n t ed in Paper II i n d i c a t e t h a t t h i s type of

r e a c t i o n occurs in a l l composi tes s t u d i e d , as both pH and sodium ion

c o n c e n t r a t i o n i n c reased in t h e s to r a g e water o f a l l the composites

with t im e .

According to the SEN i n v e s t i g a t i o n ( I I ) , the u n t r e a t e d f i l l e r s in

t h e BIS-GMA/TEGDMA composi tes showed th e f i r s t s ig ns of h y d r o l y t i c

d e g r a d a t io n , whi le the u n t r e a t e d f i l l e r s in the PMMA composi tes showed

t h e most d ramatic changes a f t e r a s ix-month exposure to w a te r . This

d i f f e r e n c e might be expla ined as a r e s u l t o f d i f f e r e n c e s in t h e

d i f f u s i o n r a t e s o f w a te r , sodium, and hydroxyl ions in the two r e s i n s .

38

According to Paper I I I and a previous i n v e s t i g a t i o n by Braden (1964) ,

the exchange r a t e of water through the PMMA matr ix i s f a s t e r than t h a t

th rough th e BIS-GMA/TEGDMA r e s i n . The leaching of sodium from the

PMMA composite should then be i n i t i a t e d f a s t e r than in the BIS-

GMA/TEGDMA compos i te . Even though the g ene ra t io n o f hydroxyl ions as

a r e s u l t o f sodium ion-hydrogen ion exchange might be f a s t e r in t h e

PMMA m a t r i x , the hydroxyl ions might a l s o d i f f u s e away f a s t e r from the

f i l l e r s u r f a c e in t h i s m a te r i a l than from corresponding reg ions of t h e

BIS-GMA/TEGDMA composi te . Such a d i f f e r e n c e in hydroxyl ion d i f f u s i o n

could mean t h a t even though the hydroxyl ion format ion i n s i d e a

BIS-GMA/TEGDMA composite i s s lower , th e c r i t i c a l pH f o r i n i t i a t i n g

h y d r o l y t i c de gr ada t ion could be reached sooner i n s i d e t h i s composi te .

When the c r i t i c a l pH va lue has been reached i n s i d e th e PMMA

composi te , the a u t o c a t a l y t i c p rocess and f i l l e r de gra da t io n could very

well proceed more r a p i d l y in th e PMMA composi te than in the

BIS-GMA/TEGDMA composi te due to the f a s t e r d i f f u s i o n of water in t h e

PMMA r e s i n . This may be demons tra ted through r e a c t i o n s (2) and (3)

which show t h a t water i s n ecessary f o r a r e g e n e r a t i o n of hydroxyl io ns

( C h a r l e s , 1958). Ac co rd ingly , the f a s t e r the water molecules reach

the f i l l e r s u r f a c e , the f a s t e r the a u t o c a t a l y t i c p rocess w i l l p roceed

when i t has been i n i t i a t e d .

The r e s u l t s p r esen ted in Paper I I I showed t h a t the d i f f u s i o n

c o e f f i c i e n t o f wat e r in BIS-GMA/TEGDMA was approximate ly o n e - te n t h

t h a t o f PMMA. Regarding th e changes in s o r p t i o n and d e s o r p t i o n

c o e f f i c i e n t s between the f i r s t and second cyc le a t 60°C, i t has been

hypothes ized t h a t s t r e s s e s induced dur ing sw el l in g and dehy d ra t i o n

induced s u r fa c e c racks (Rosen, 1961). However, no a t te m pt s were made

39

t o exp lo re t h i s th e o ry through SEM a n a l y s i s . F ur th e r s t u d i e s o f t h i s

type should be performed to de te rmine whether or not such c rack forma

t i o n o c c u r s , as such c racks would f a c i l i t a t e wear o f th e mat r ix

m a t e r i a l .

In Paper I I I , i t was f u r t h e r shown t h a t the l i n e a r swe l l i ng due

t o water s a t u r a t i o n was as high as 1.08%. The sw e l l i ng process may

have e i t h e r a p o s i t i v e or ne ga t ive impact on th e performance of a

composi te m a te r i a l (Asmussen & Jo rge nse n , 1972; Hembree, 1983). Water

sw el l ing may compensate f o r th e s e t t i n g c o n t r a c t i o n (Asmussen &

J 0 rg en se n , 1972). However, ex c e s s iv e sw e l l i ng could induce s t r e s s e s

a t the r e s t o r a t i on-enamel i n t e r f a c e , c o n t r i b u t i n g to debonding and

margina l leakage (Hembree, 1983).

Based on the d i f f u s i o n c o e f f i c i e n t s a t 37°C and 60°C, and assum

ing t h a t the Arrhenius r e l a t i o n s h i p i s v a l i d in t h i s i n t e r v a l , i t i s

p o s s i b l e to e s t i m a t e the a c t i v a t i o n energy f o r wa ter d i f f u s i o n in t h e

BIS-GMA/TEGDMA r e s i n . The c a l c u l a t e d a c t i v a t i o n energy va lue i s

approximate ly 10 kca l /mole . This va lue i s a lmost i d e n t i c a l to t h e

v a p o r i z a t i o n energy of water a t th e se tempe ra tur es and may i n d i c a t e

t h a t the energy needed to produce wa ter d i f f u s i o n through a BIS-

GMA/TEGDMA r e s i n i s consumed p r i m a r i l y by water v a p o r i z a t i o n (Moore,

1968). These r e s u l t s imply t h a t the d i f f u s i o n of water in the i n v e s

t i g a t e d BISGMA/TEGDMA r e s i n i s a p rocess where water molecules pene

t r a t e as water vapor in to th e network where they a r e a t t r a c t e d by

s t r o n g p o l a r groups . When an o th e r a c t i v a t e d water molecule reaches

such a hydrogen-bonded water molecule , i t can d i s p l a c e the p r e v i o u s l y -

a t t a c h e d water molecule . In t h i s p r o c e s s , i t s k i n e t i c energy i s l o s t ,

and th e molecule becomes bonded by the p o l a r group p re v io u s ly occupied

40

by the d i s p la c e d molecule . This model i n d i c a t e s t h a t th e d i f f u s i o n

of water through t h i s r e s i n could be q u i t e pa ss iv e regard in g cha in

r o t a t i o n s o f the r e s i n (Thomas, 1951).

In Paper IV, the leaching behavior of four commercial comp os i te s ,

Conc ise ®, P r o f i l e 0 , F o t o f i l 0 , and S i l a r 0 , was i n v e s t i g a t e d . This

s tudy was designed to de te rmine whether the h y d r o l y t i c d e g r a d a t i o n ,

which was r e p o r t e d in Paper I I , a l s o r e s u l t e d wi th f i l l e r s used in

commercial p rod uc ts . To be ab le to t e s t the hypo thes i s t h a t a g l a s s

modi fy ing element such as sodium f a c i l i t a t e s the h y d r o l y t i c

degr ad a t io n of f i l l e r s in compos i tes , two composi tes c o n t a i n i n g

s t r o n t i u m or barium were s e l e c t e d . These m a t e r i a l s , P r o f i l e and

F o t o f i l 0 , were compared to Concise0 and S i l a r ® which c o n ta in e d

r e l a t i v e l y pure S i 0^ f i l l e r . The hypo the s i s being t e s t e d was t h a t

P r o f i l e and F o t o f i l would e x h i b i t more pronounced leaching than t h e

o t h e r two m a t e r i a l s . A f te r 30 days in w a t e r , the r e s u l t s confirmed

the v a l i d i t y of th e proposed h y p o th e s i s . However, during the second

p e r io d a decrease in leach ing occurred f o r a l l m a t e r i a l s excep t f o r

S i l a r 0 . The S i l a r ® composi tes e x h i b i t e d twice as high a s i l i c o n

le a c h in g r a t e dur ing the second p e r i o d . This f i n d i n g , combined with

the f a c t t h a t the s i l i c o n was leached from the quar tz f i l l e r i n

Concise® , c r e a t e d some doubt on whether only one mechanism was

r e s p o n s i b l e f o r the leaching of g l a ss m od i f ie r s and whether breakdown

o f the s i l i c a network was the s o l e ba s i s f o r the h y d r o l y t i c

d egrada t ion of f i l l e r s in composi tes (C h a r le s , 1958; Wiederhorn &

Bolz , 1970). However, the t o t a l f i l l e r su r f a c e a rea o f th e d i f f e r e n t®

composi tes might e x pl a in why S i l a r r e l e a s e d more s i l i c o n than any®

o t h e r m a t e r i a l . Based on th e assumpt ion t h a t S i l a r has s p h e r ic a l

41

f i l l e r s G.04 /um in d i am e te r , while the d iameters of the o t h e r f i l l e r s